It depends which element you are interested in.
Following the Japanese accident there were 2 main
elements released into the air.
Iodine 131 ... this one has an 8day halflife so it is pretty well gone
after 3 months Cesium - 137 This has a 32 year halflife so it will be around for a couple hundred years.
This means some areas may need cleaning up of Cesium-137
The radiation in the nuclear fuel itself can last for thousands
of years....this is the biggest problem to deal with.